The prehydrolysis-sulfate (Kraft) cooking for the production of special pulps having a high content of alpha cellulose was developed in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968. The basic idea is to remove as much hemicellulose as possible from cellulose fibers in connection with delignification, so as to obtain a high content of alpha cellulose. This is essential because the various end uses of such pulps, dissolving pulp for instance, do not tolerate short-chained hemicellulose molecules with a randomly grafted molecular structure.
A separate prehydrolysis step permits the desired adjustment of the hydrolysis of hemicelluloses by varying the hydrolysis conditions. In the prehydrolysis-kraft cooking process the necessary delignification is not carried out until a separate second cooking step. The prehydrolysis is carried out either as a steam or water phase prehydrolysis, or in the presence of a catalyst. In the former “steam” processes, organic acids liberated from wood during the process establish the necessary pH conditions and perform a major part of the hydrolysis, whereas in the latter “water” process, small amounts of mineral acid or sulfur dioxide may be added to “assist” the prehydrolysis. In the prehydrolysis stage carried out in a steam phase, often called autohydrolysis, direct steam is introduced to the chip column in the digester. Conventionally, autohydrolysis is established at some 30-40° C. higher temperature than in liquid filled hydrolysis.
Conventionally after prehydrolyzing the cellulosic material in a reactor, the hydrolysate and the prehydrolyzed cellulosic material are neutralized in the reactor with alkaline neutralizing liquor so as to produce neutralized hydrolysate and neutralized prehydrolyzed cellulosic material. There is hydrolysate both in the free liquid outside the chips and also trapped and immobilized inside the chips. In Bio Pulping, as much as possible of the hydrolysate can be recovered before the neutralization step in order to be able to utilize the carbohydrates released in the prehydrolysis as an additional product from the mill. A separate washing stage, in which the digester is first filled up with a washing liquid and then the liquid containing the carbohydrates is removed from the digester, can be used between the prehydrolysis and cooking stages. Conventionally, both the liquid filling of the digester as well as removal of the dissolved carbohydrates are done by a displacement process using heated wash liquors, all in order to maintain the temperature of the cellulose material.
EP 2430233 discloses another method to recover the hydrolysate from a steam phase prehydrolysis. In EP 2430233 hot water is introduced into the digester after prehydrolysis at top and bottom and subjected to internal circulation while filling the digester. The water filling may be continued until the entire chip volume inside digester is drenched in water. The hot water is heated to the intended temperature and stored in hot water accumulator before usage. The heating is done up to a temperature close to the temperature of the hydrolysis.
Also, a sequence of multiple displacement liquors may be used in a sequence during displacement, and one such sequence is shown in EP796367. After a prehydrolysis at some 170° C. is the hydrolysate neutralized by displacing a hot white liquor pad through the digester at some 155° C., and thereafter is kraft cooking commenced using spent cooking liquor at some 148° C. in a first phase. A problem here is that the very first portion of the hot white liquor pad that meets the hot acidic chips both is heated by the chips and due to exothermic reactions further elevate the temperature in the white liquor pad, and this while the alkali content is consumed. Thus, the last upper volume of the digester content will be exposed to a hot and alkali depleted white liquor pad that is not able to end the prehydrolysis. This will cause an extended prehydrolysis in upper part of digester in comparison to lower part, and the difference in prehydrolysis effect between upper and lower part of digester could be some 17-150%.
Similar displacement using a white liquor pad, added in volume at some 30 m3 in a digester with a total volume about 300 m3, is also disclosed in EP2567023, but ahead of a CCE-filtrate added in volume at some 130 m3. Sequential displacement from bottom is also disclosed for hot black liquor filling as well as final liquor displacement. All displacement liquors used having an isothermal temperature when adding them to the digester.
While the processes hereto has been optimized for maintaining the established heat value in the digester, i.e. avoiding losses of heat value in the process, most implementations has used excessive heating of process liquors added after a hot treatment stage. The objective in this excessive heating has been to maintain the temperature of the content of the digester high, avoiding the losses that may be at hand if the digester is first elevated to a high process temperature, then lowered in temperature, followed by heating again to establish a higher temperature. Each such swing in temperature leads to heat losses per default, even if heat recovery is implemented after each phase.
The system has thus been designed with large accumulators for storing the heated process liquors, which accumulators are equipped with circulation systems and heat exchangers in order to heat the liquors to this elevated temperature before use at the specific treatment phase.
What is also seen in the prior art is that even if these high temperature liquors are used to end a treatment phase, the digester content is subjected to different H-factor exposure as of content close to bottom VS content close to top, and especially if the temperatures differ between phases. Using an isothermal displacement liquor after a hot treatment phase, and a somewhat colder displacement liquor to the bottom of the digester, will impose a larger cooling effect on the digester material contained in the bottom VS the digester material contained in the upper part. This due to that the displacement liquor will be heated by the digester material during displacement and in some cases due to exothermic reactions. At the instant where the displacement front reach the top is the temperature of the free displacement liquor often more than 20-40° C. higher than the temperature of the displacement liquor last added to the bottom. Now, the temperature profile may be even out afterwards by circulation, but the harm has already been done at the moment this temperature profile is obtained.